Typically, an array of filter units is disposed above or to the side of a clean room. The term "clean room ceiling" herein refers to either an overhead array, or a to-the-side, sidewall array of filter units. In either case the room air is collected, recycled, and brought under pressure to the filter units. A volume of air to be filtered is typically pressurized on the upstream side of the filter unit to enable flow through the filter media. The term "plenum" herein refers to that space in which air is at a relatively higher pressure before flow through the filter media than on the outlet side of the filter. In a large filter installation the plenum is a large volume space above the clean room ceiling. In a small installation, such as a bench apparatus, the filter unit is typically combined in a housing with a fan, which serves to pressurize the air for through-filter media flow. The plenum-contained, pressurized air passes through the rear face of the filter units, through the filter media within the filter units and to the front face of the filter units for reintroduction into the clean room or toward the bench.
Because the filter units are of relatively limited size, a typical unit being only two feet by four feet, and the frequent need to cover a greatly extended area over a clean room, often the size of several football fields, multiple ones of the filter units are mounted adjacent one another in a laterally and longitudinally extended array, and the array sealed against leakage of unfiltered air from the plenum. The filter array mounting and sealing challenge has been met in the past by erecting extensive grids, suspended from above, onto which the filter units are placed. These grids often take the form of troughs with the individual filter units being fitted into a given rectangle of the grid with a projecting portion of the filter units being immersed in a gel sealant carried in the trough. The sealant and filter unit portions cooperate to block air flow from the plenum from entering the clean room except through the filter units. The grid troughs have a certain width, which increases the lateral spacing between filter units, ironically increasing the paths for sideflows of unfiltered air. In addition, the trough width reduces the proportion of filtered air area in the clean room ceiling, and induces turbulent air flow between the filter units, causing vibration.
Gel sealants to be effective must retain some elasticity or capacity to flow, despite years in place, so as to continually bear against the filter unit portions in the trough despite aging of the gel and vibration of the filter units. Replacement of the HEPA filter units as they become inefficient is a burdensome task. The individual units must be lifted up and away from the gelatinous mass in the troughs sufficiently to clear the trough edges, tipped or somehow angled so as to pass through the grid opening defined by the troughs, and carried out of the grid. Clinging gelatinous sealant can be messy. The installation of the new filter unit repeats these steps in reverse. More gel sealant may need to be added; these gels are quite expensive.
In U.S. Pat. No. 4,883,511 to Gustin et al, a clean room ceiling system is shown which uses gel sealants to seal filter units held in a vertically staggered array to support each other without use of a planar grid of troughs. In U.S. Pat. No. 5,329,739, the patentee Madl describes a clean room filter system in which clusters of four filter units are supported together at the intersection of their respective corners by a disk overlapping each corner, the disk depending from a support surface such as a ceiling. Gel sealant carried on sealing strips seals the filter units against unfiltered air leakage. Mini grid assemblies have been proposed in which four filter units are supported on a common grid section, the grid section being supported from above by rods at corners of the grid section in German No. 3719734 A. In the parent application of this application, now U.S. Pat. No. 5,871,556, individually supported filter units were first disclosed; these used a suspension bracket within the filter unit to anchor to a fastener that depended from the ceiling or other support so that the filter units could be simply pressed into place from below the ceiling without disturbing the other filter units adjacent.